Hitherto, high-hardness sintered bodies using cubic boron nitride have been known. For example, Japanese Patent Laying-Open No. 53-077811 (Patent Document 1) discloses a cBN sintered body containing 20 to 80 volume % of cBN and containing, as the balance, a Ti ceramic binder. However, the sintered bodies do not give a satisfactory tool life span against high-efficiency cuttings, such as interrupted cutting, and heavy-cutting/high-speed cutting. Thus, in order to improve the toughness or the heat resistance of the sintered bodies, the binder is improved, the cBN particles are covered with a ceramic material to strengthen the binding force and improve the dispersibility, and others are attained.
Sintered bodies high in toughness wherein the Ti ceramic binder in Japanese Patent Laying-Open No. 53-077811 (Patent Document 1) is improved are disclosed in Japanese Patent Publication No. 60-014826 (Japanese Patent Laying-Open No. 56-069350 (Patent Document 2)), Japanese Patent Publication No. 61-054857 (Japanese Patent Laying-Open No. 61-179847 (Patent Document 3)), Japanese Patent Publication No. 61-054858 (Japanese Patent Laying-Open No. 61-179848 (Patent Document 4)), and Japanese Patent Laying-Open No. 05-287433 (Patent Document 5), and others. The sintered bodies disclosed in these documents 2 to 5 are high in toughness, and are suitable for interrupted cutting of hardened steel or the like.
Moreover, Japanese Patent Laying-Open No. 10-182242 (Patent Document 6) discloses a sintered body wherein the peripheries of cBN particles are coated with a binder to hold the cBN particles by the binder, whereby the cBN particles are not bonded directly to each other.
U.S. Pat. No. 5,639,285 (Patent Document 7) discloses a sintered body wherein a Ti ceramic binder is improved to strength the binding force between cBN particles and binder particles, thereby bonding the cBN particles mutually to each other.
Furthermore, Japanese Patent Laying-Open No. 07-252580 (Patent Document 8) and Japanese Patent Laying-Open No. 07-252581 (Patent Document 9) each disclose a sintered body wherein Si is incorporated into a binder.
As described above, sintered bodies usable in interrupted cutting of high-hardness hardened steel are supplied by improving the toughness of cBN sintered bodies by the following: an improvement in a binder itself or an improvement in the sintered body microstructure by avoidance of contact between cBN particles or reversely by positive bonding of cBN particles to each other, or some other matter.    Patent Document 1: Japanese Patent Laying-Open No. 53-077811    Patent Document 2: Japanese Patent Publication No. 60-014826 (Japanese Patent Laying-Open No. 56-069350)    Patent Document 3: Japanese Patent Publication No. 61-054857 (Japanese Patent Laying-Open No. 61-179847)    Patent Document 4: Japanese Patent Publication No. 61-054858 (Japanese Patent Laying-Open No. 61-179848)    Patent Document 5: Japanese Patent Laying-Open No. 05-287433    Patent Document 6: Japanese Patent Laying-Open No. 10-182242    Patent Document 7: U.S. Pat. No. 5,639,285    Patent Document 8: Japanese Patent Laying-Open No. 07-252580    Patent Document 9: Japanese Patent Laying-Open No. 07-252581